IS THE 670 KM PHASE-TRANSITION ABLE TO LAYER THE EARTHS CONVECTION INA MANTLE WITH DEPTH-DEPENDENT VISCOSITY

The effect of a viscosity stratification on phase change dynamics have been investigated with axi-spherical convection models. As in previou s studies with a constant viscosity mantle an intermittent layering ap pears for a Clapeyron slope from -2 MPa/K to -3 MPa/K. A viscosity inc rease in lower mantle requires a more negative Clapeyron slope to prod uce the layering. This shift is sensitive to the mechanical boundary c ondition. With a viscosity contrast of 30, a no-slip top condition doe s not lead to layering in the range of the possible values for the Cla peyron slope. With a free-slip condition, the threshold is at -4 MPa/K . Just below this threshold, a whole mantle circulation driven by a cy lindrical hot plume coexists with layered mantle domains over several billion years.